Given n non-negative integers representing an elevation map where the width of each bar is 1. Write a program to compute how much water it is able to trap after raining.
The above elevation map is represented by array [1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.
Input: [0,1,0,2,1,0,1,3,2,1,2,1] Output: 6